Tillerson's ouster came one day after he publicly broke from the White House and blamed Russia for ordering a chemical attack against a former Russian spy in the UK.

The White House press secretary Sarah Huckabee Sanders said Monday that the attack was "reckless, indiscriminate, and irresponsible" but declined to specifically blame Russia as the perpetrator. Trump has not yet publicly commented on the matter.

Later in the evening, Tillerson told reporters the attack "clearly" came from Russia and then expressed shock at the Kremlin's actions.

"It's almost beyond comprehension that a state, an organized state, would do something like that," he told reporters aboard a US airplane on a trip to Africa.

"A non-state actor, I could understand. A state actor — I cannot understand why anyone would take such an action," he said.

When asked whether the incident would prompt a mutual defense response with the UK, Tillerson said that "it certainly will trigger a response. I'll leave it at that."

The Washington Post reported Tuesday that Trump had asked Tillerson to resign from his post last Friday, and that Tillerson cut his Africa trip short on Monday so he could return to Washington. A top Tillerson aide told a different story Tuesday, saying Tillerson had "every intention of staying" and that he had not spoken to Trump about his removal.

Speaking to reporters outside the White House shortly after Tillerson's ouster, Trump issued his first public statement of solidarity with the UK, saying "it sounds to me like they believe it was Russia, and I would certainly take that finding as fact."

Tillerson's statements about Russia on Monday mark one of his most vehement condemnations of the Kremlin to date. The former secretary of state attracted significant scrutiny after he was nominated for the post last year because of his prior relationship with Putin, whom he has known since at least 1999.

Putin also personally awarded Tillerson with Russia's Order of Friendship medal in 2013, the highest honor bestowed in foreign citizens.

As secretary of state, Tillerson has criticized Russia for meddling in the 2016 US election, as well as for its support of Syrian President Bashar al-Assad, who has been accused of carrying out widespread chemical attacks against Syrian citizens in the country's ongoing civil war.

Sergei Skripal, a 66-year-old former Russian military intelligence officer, and his daughter Yulia were poisoned by the nerve agent Novichok, which was developed by the Soviet Union in the 1970s and 1980s, earlier this month. British Prime Minister Theresa May said Monday that it was "highly likely" Russia was behind the attack, and gave the Kremlin a deadline of late Tuesday by which to provide a "credible response" to the UK's findings.

Moscow scoffed at the allegations and called them a "circus show." Russian President Vladimir Putin also deflected questions about the incident during an interview with BBC on Tuesday.

Reporter Steve Rosenberg cornered Putin at an agriculture event in Krasnodar, a region in southern Russia. He asked: "Is Russia behind the poisoning of Sergei Skripal?"

"Look, we're busy here with agriculture. As you can see the aim is to create good conditions for people's lives and you ask me about some tragedies," Putin said. "Get to the bottom of things there first, then we'll talk about this."